Welcome to the Microsoft Q&A forum and thank you for posting a detailed question here.
Since the simulated clients will be present on Azure and will have an Azure public IP assigned to them the traffic will go over Microsoft WAN Network and I do not think there is any option available here to direct the traffic via ISP.
You will have to generate traffic via a non-azure Public IP and use Routing over public Internet
option to simulate the real-life scenario mentioned above.
Just sharing as an FYI, based on your requirements and services provided. I was wondering if you have considered of implementing a CDN to provide your services across the world. If interested, you can check out the Azure Front Door Service where you can leverage over 118 edge locations across 100 metro cities connected to Azure using a private enterprise-grade WAN and improve latency for apps by up to 3 times.
If interested, you can also check out Load Testing service in Azure: https://video2.skills-academy.com/en-us/azure/load-testing/overview-what-is-azure-load-testing
Hope this helps! Please let me know if you have any questions. Thank you!
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